1. Field of the Invention
The present invention relates to a fixed form image reading apparatus and a method for reading a fixed form image that has a regular shape like a hologram or a two-dimensional code.
2. Description of Related Art
Recently, two-dimensional codes that can store more information in a smaller area than one-dimensional codes (so-called bar codes) are used. The two-dimensional code can store a large capacity of information compared with a conventional bar code, and it has little limitation of reading direction because it has a position detection mark.
The reading apparatus for reading the two-dimensional code optically is equipped with an image sensor such as a CCD, a CMOS, a photo diode or the like for detecting an optical signal and converting it into an electric signal, a decode circuit for converting the electric signal into code information, and optical elements including a collimator lens, an objective lens, a mirror, a polygon mirror and the like. In addition, there is a case where it is equipped with a light source that projects light to the two-dimensional code as necessity.
The reading apparatus takes an image of the two-dimensional code by the image sensor and decodes its image data by a decoding portion so that data stored in the two-dimensional code is extracted.
The reading apparatus is vulnerable to influences of dark current of the image sensor or noises. In order to remove such influences, there is disclosed an imaging apparatus in which an image is taken in the state where its shutter is closed (i.e., the state where incident light of the image sensor is shut out), and correction is performed based on image data obtained from the image sensor in the state where the shutter is closed (see JP-A-H8-307775, JP-A-2001-94882 and the like, for example).
The fixed form image such as the two-dimensional code or the like has a marker for detecting its position (direction or place). The marker is formed so that a large luminance change is generated, and the marker is detected based on the luminance change for detecting the position.
However, if luminance changes largely in the image sensor, the part of the image data (the part where the luminance changes largely) is formed with gradation. As a result, there is a case where an actual fixed form image is different from an image obtained by the image sensor. The fixed form image is formed in accordance with a predetermined rule. If the obtained image is different from the actual fixed form image, data stored in it may not be extracted or obtained data may not be correct data even if it is extracted.